The present invention relates to a high-efficiency turbine, particularly for exploiting wind power in auxiliary power sources for aeronautical applications.
Wind-driven turbines are known which are installed on aircraft and are intended to supply power to the onboard systems in emergency conditions or for special applications, by using the kinetic energy of the surrounding air as it flows relative to the aircraft.
Such turbines, known as "Ram Air Turbines", generally have a small number of radial vanes (usually two or four) and are connected to the shaft of an alternator and/or to the shaft of a pump to generate electric and/or hydraulic power.
In view of the extensive aerodynamic range within which these turbines are used, from the minimum speeds of the aircraft up to the sonic range at altitudes comprised between 0 and 15,000 meters, and in view of the relative uniformity required for the speed of the alternator, said turbines have a device for controlling their rotation rate. Said device is generally composed of an element responsive to the rotation rate (constituted for example by a hydraulic pump or by a Watt pendulum with centrifugal masses), an element for varying the rigging angle of incidence of the vanes (that is the angle formed by the vanes with a plane perpendicular to the turbine axis), for example a hydraulic cylinder, and a feedback element which completes the adjustment circuit.
Said adjustment devices are structurally complicated and are often scarcely reliable; if hydraulic components are used, their operation is furthermore not constant as the environmental parameters vary, such as for example when the temperature varies.
Turbines are also known in which the rotation rate adjustment device does not use hydraulic components but exclusively mechanical components. More particularly, these turbines generally have four vanes, of which two are keyed on the central body of the turbine with a fixed incidence angle while the other two have a variable incidence angle. Eccentric masses are used to adjust the rotation rate of the turbine; said masses act directly on the movable vanes, in contrast with elastic means, varying their incidence angle, while the feedback element is constituted by mechanical components (generally a connecting rod-crank assembly) which are precisely connected to the other elements which compose the adjustment device. In practice the adjustment device only controls two vanes, which are oriented about their radial axis, while the other pair of vanes, which is arranged at a right angle, remains fixed in a position optimized for minimum operating speeds; at higher speeds the adjustment is therefore provided by balancing between the excess motive torque provided by the fixed vanes and the resistive torque provided by the two movable vanes, which progressively reduce their supporting contribution as the speed rises until they act as aerodynamic brakes.
Though this known kind of turbine is structurally simpler, more reliable and insensitive to temperature variations with respect to the previously described turbines, they have some disadvantages.
Said turbines in fact have an acceptable efficiency with low relative speeds of the motive fluid, but at high speeds their efficiency is considerably penalized. Though such a condition is acceptable when using the turbine as an emergency generator, which in any case uses a non-relevant part of the overall available energy, it entails excessively low energy efficiencies within the scope of its systematic use as decentralized generator.
Known turbines furthermore have a relatively high aerodynamic resistance which affects fuel consumptions at high speeds and requires the use of adequately sized fixing structures which affect the overall weight of the aircraft. Said turbines furthermore produce aerodynamic disturbances for the aircraft, again due to their relatively high aerodynamic resistance.